Field of the Invention
This invention is concerned with control of microelectronic assembly processes, and in particular with measuring the thickness of palladium layers in surface finishes.
Description of the Related Art
Surface finishes comprising a palladium outer layer are widely used by the electronics industry to protect copper substrates from oxidation so as to ensure solderability and wire bondability of bonding wire, device leads, leadframes, printed wiring boards and surface pads. A nickel underlayer is often used to prevent cracking of the palladium coating and exposure of the copper substrate. It is necessary to control the thickness of the palladium layer so as to provide adequate oxidation resistance to the substrate with a minimum amount of palladium, which is a relatively expensive metal.
Palladium thickness is typically measured in the prior art by x-ray fluorescence (XRF), which requires relatively complicated and expensive instrumentation and is difficult to apply to the odd geometries and small specimen areas typical of microelectronic devices. A recent study by S. Dill and V. Rössiger [Circuit World 37(2), 220-226 (2011)] found that the well-established XRF instrumentation with proportional counter detectors is not very suitable for measuring the thickness of thin palladium coatings (<0.1 μm) due to poor energy resolution of the proportional counter-tubes. These workers also found that semiconductor detector systems provide more reliable XRF thickness measurements with significantly higher accuracy but require special x-ray optics (polycapillaries) for small measuring spots (e.g. 0.15 μm). In any case, the effect of the composition of the substrate base material must be taken into account by the XRF software evaluation algorithm for each measurement. A global base material subtraction performed prior to the measurement can provide better repeatability but can also lead to incorrect absolute values of the thickness [S. Dill and V. Rössiger, Circuit World 37(2), 220-226 (2011)].